Several different types of mount assemblies are available to isolate component vibrations, such as produced during operation of automobile and truck engines and transmissions. A type of mount commonly used today is the solid elastomeric block fabricated, for example, of natural or synthetic rubber integrally molded with rate plates and including metal mounting platforms and bolts for attachment. In order to support an engine or the like, a number of these solid elastomeric mounts are attached between the mounted component and the vehicle frame.
When utilized in this manner, the mounts are required to support the engine or other component in the manner of a spring while also functioning as a shock absorber element to dampen vibrations. These dual functions often require conflicting operating characteristics and, accordingly, a trade-off of different design considerations. As a result, the mount is a subject of compromises in design, providing neither optimum motion control nor isolation properties.
More particularly, while solid elastomeric mounts may be designed to operate in a generally satisfactory manner, engineers are limited in what may be achieved. For example, many elastomeric or rubber-like materials selected for their load support properties inherently have high stiffness limiting their ability to isolate certain objectionable vibratory inputs, such as those particularly disturbing to occupants in a modern, downsized vehicle having a unitized vehicle body and frame construction. Accordingly, there is a need for improvement in balancing these conflicting dynamic requirements of load support/motion control and vibration control.
High damping is generally desired to reduce low frequency, high amplitude vibrations of the mounted component. One approach to increase the damping coefficient is by the selection of certain polymers and the use of additives. Thus far, however, this approach has proven unsatisfactory because of accompanying adverse effects on other properties of the material including loss of durability. Furthermore, this approach produces large damping for all vibratory inputs regardless of frequency or amplitude. This is undesirable in an engine mount, particularly in the low amplitude and high frequency ranges, as it leads to the increased transmission of noise.
As a result of these difficulties, there has been an ongoing effort in progress to produce a cost-effective means of providing prescribed and varying mount properties, tuned to selectively suppress vibrations of particular problem amplitudes and frequencies. In the case of an engine mount, this calls for substantially increased damping of certain low frequency and high amplitude vibrations. This needs to be accomplished, however, while maintaining relatively reduced damping and stiffness in the case of low amplitude and high frequency vibrations so as to provide desired noise suppression. Stated another way, the bottom line is the provision of a mount assembly having a dynamic rate meeting load support requirements, as well as the needs of vibration control including (1) the suppression and isolation of noise, i.e. engine-produced vibrations typically lying in a frequency range of 20 to 400 Hz, and (2) effective damping of larger engine displacements or shake, i.e. the effect on the vehicle body of the coupling of high amplitude vibrations of the body, engine and suspension, typically in the frequency range of 8 to 16 Hz. Furthermore, the desired mount characteristics should be achieved in a way that does not compromise other design considerations, such as prescribed stiffness ratios along the major axis for load support, and prescribed mount configurations to suit packaging space limitations.
In order to achieve this end, various mount assembly designs have been proposed combining hydraulic damping and basic elastomeric mount features. Hydraulic/elastomeric mounts of the type disclosed in, for example, U.S. Pat. No. 4,588,173 to Gold et al., issued May 13, 1986, and assigned to the assignee of the present invention, include features, such as selectively placed damping chamber sections, selective length orifice tracks and decoupler(s), to passively tune the isolation and damping characteristics. Thus, these mount assemblies have met with considerable success compromising the conflicting objectives of load support and engine vibration control.
Other hydraulic elastomeric mounts, such as disclosed in U.S. Pat. No. 4,783,062 to Hamberg et al., issued Nov. 8, 1988, and also assigned to the assignee of the present invention, utilize an adaptive approach. In Hamberg, et al., the mount includes a gate responsive to sensed vibrations to control the flow of hydraulic fluid along the orifice tracks, thereby actively tuning the dynamic characteristics of the mount to even better optimize firm engine support and noise/vibration control over a broader range of operating conditions. As a result of these developments, hydraulic/elastomeric mounts have grown in favor in recent years. They have generally succeeded in being more versatile and responsive than solid elastomeric mounts so as to meet the modern needs of vehicle manufacturers.
While effective for their intended purpose, it must be appreciated that hydraulic/elastomeric mounts are not necessarily the ultimate in mount design and that alternative designs with similar enhanced operating characteristics need to be considered. One possible alternative design is to produce an elastomeric mount assembly without hydraulics, but with active vibration control. The approach is to gain more responsiveness to vehicle operating conditions, and thus better equip the mount to isolate troublesome vibrations and reduce noise, in a manner not possible with past solid elastomeric mount designs. Such an elastomeric mount assembly should be able to provide remarkable dependability over an extended service life.